1. Field of the Invention
The present invention relates generally to multi-node sensor systems. More specifically, the present invention relates to a system, sensor nodes, program product, and related methods to monitor the health of structural components and to minimize data collisions between nodes.
2. Description of the Related Art
Various types of platforms such as, for example, aircraft structural components, aircraft skins, or other related components, when in operation are subjected to various environmental conditions such as stress and strain, exposure to temperature extremes, and/or significant vibration energy. Due to the various environmental conditions such components can suffer material degradation over time.
Structural health monitoring helps promote realization of the full potential of such components. Remotely position sensors (sensor units or nodes) have been installed adjacent to such structures/components to monitor various parameters such as, for example, strain levels, stress, temperature, pressure, or vibration level to help manage physical inspection schedules, maintenance schedules, to help predict material failure, and generally monitor the “health” of such components. Such sensors have been provided a dedicated power supply such as power obtained through conductors, e.g., wires, connected to the aircraft electrical system or through chemical batteries. Such wiring can undesirably result in increased weight and complexity of the component being monitored and/or of the associated structure or component and are subject to damage or breakage requiring extensive repair costs and down time. Depending upon available space, batteries can be inappropriate due to their size. Batteries can also have a limited service life and therefore typically require periodic inspection and/or replacement, are often positioned in locations difficult to reach, and often require costly disassembly and reassembly of the sensor or component to perform service on the battery. Further, batteries may not be suitable due to environmental constraints, i.e., temperature changes often affect battery performance.
Some more recent structural health monitoring systems include fiber-optic sensors connected to a network of fiber-optic conductors to form an interrogation system. Such fiber-optic conductors, as with electrical conductors, can significantly raise the complexity of the component and/or deployment of the sensor system. Other structural health monitoring systems include self-powered sensors attached to or embedded within the components to be monitored that can reduce dependence on batteries or any other external power source. Such sensors can be relatively small in size and can utilize, as a power source, energy obtained or otherwise transmitted through the component or structure being monitored. Such devices can include those known as micro-electro-mechanical systems (MEMS). This type of sensor can typically consume very low amounts of power in the microwatt range. Other such devices can also include those known as piezoelectric devices. Some related piezoelectric devices can be in the form of actuators which can apply a force on the skin structure to dampen detected vibrations.
Other data capturing systems, such as that employing radiofrequency identification, can also be used in health monitoring. Such systems can include both active and passive wireless sensors (transponders and sensor elements) attached to or positioned within the component or structure to be monitored. The active wireless sensors can actively or passively collect and provide a continuous or intermittent stream of sampled raw data indicating parameters of the component or structure being monitored. The sensor data is typically collected by a central collector or by a series of intermediate collectors which provide such data to a central collector. The passive wireless sensors can also collect a continuous or intermittent stream of sampled raw data indicating parameters of the component or structure being monitored. The passive sensors, however, do not actively transmit such data, but instead receive energy from, for example, a mobile vehicle or handheld base device or reader positioned adjacent each wireless sensor, which provides power to extract the sensor data. The passive wireless sensors are most often utilized in applications where ultra low power communication is desired. In such passive systems, a reader can transmit a signal to each passive wireless sensor to power the sensor and to transmit a request for data. The request for data can be in the form of a request directed at a specific wireless sensor or the mere provision of energy in an applicable frequency band. In response to the signal from the reader, the wireless sensor can vary impedance of a wireless antenna, which the reader detects, to thereby receive raw sensor data.
There are two generally accepted methods of extracting data from such passive wireless sensors: inductive coupling and backscatter coupling. In both inductive coupling and backscatter coupling, the antenna of the reader generates a strong high frequency electromagnetic field which can penetrate the structure being monitored to interact with the wireless sensor antenna. Specifically, in interactive coupling, an electromagnetic field is radiated outward from the antenna of the reader to the passive sensor antenna. A portion of such field engages the antenna such that a voltage and a current are generated in a coil of the antenna, with a portion of the induced voltage rectified to provide direct current power to the sensor processor. The antenna coil of the passive sensor and a capacitor form a resonant circuit having a resonant frequency which corresponds to the transmission frequency of the reader. Basically, when the passive sensor is positioned within the magnetic field produced by the reader antenna, the combination of the reader antenna and the passive sensor antenna form a transformer-like inductive coupling, which allows the passive sensor to draw energy from the reader, which can be detected by the reader. By modulating this load on the passive sensor antenna, an amplitude modulated signal can be formed within the reader which can represent the data being transferred. Alternatively, by modulating the load at a sub-carrier frequency and by modulating the sub-carrier frequency using amplitude, frequency, or phase shift keying, a modulated signal having two sidebands can be formed within the reader which can represent the data being transferred.
In backscatter coupling, as with inductive coupling, an electromagnetic field is also radiated outward from the antenna of the reader to the passive sensor antenna. A portion of such electromagnetic field not attenuated induces a voltage in the passive sensor antenna. A portion of the induced voltage is rectified to provide direct current power to the sensor processor. Rather than form a transformer-like coupling, however, the passive sensor antenna is configured such that a portion of the incoming energy is reflected by the passive sensor antenna. Such reflected or backscattered energy can then be received by the reader antenna. The efficiency with which the antenna reflects such energy coincides with its reflection cross section. By modulating a load across the passive sensor antenna, the strength (amplitude) of the reflected signal at a given frequency can be modulated to represent the data being transferred.
In structural health monitoring applications or other applications in which a large number of wireless sensors may be in close proximity to each other, many of the passive communication schemes, due to limitations on available power, rely on inductive coupling and backscatter coupling, such as that described above, and thus, do not have the ability to be switched off. If a large number of passive sensors are in close proximity to each other, collisions in the physical layer are likely to occur, thus making data communication very inefficient. Various techniques have been used in active communication schemes to avoid collisions. Some of these techniques do not have an equivalent in passive communication schemes. Others require processing and power to function, both of which add to the complexity of the communication system. For active communication schemes, this problem has been addressed through the use of multiple access schemes such as time division, frequency division and code division multiple access-type communication schemes which utilize a spread spectrum and encoded data in a pseudorandom digital sequence. This problem has also been addressed by assigning individual unique identification numbers to each sensor. The reader can then individually address each sensor to request data. For passive communication schemes, this problem has been addressed through use of assigning each passive sensor a separate narrow band resonant frequency band.
Recognized by the applicant is the need to provide a passive sensor communication system that provides a multiple access communication scheme that can ensure that communication collisions between the sensors is managed and kept to a level that allows for usable communication to take place. Also recognized is the need to provide a passive sensor communication system that provides a multiple access communication scheme that is low power, and that has less complexity than conventional multiple access methods.